Poorer breast cancer (BrCa) prognosis among African Americans (AAs) compared to other racial and ethnic groups in the United States is likely multifactorial, including tumor characteristics, socio-economic factors and poor access to care. While there is strong evidence that obesity and obesity-related comorbidities increase BrCa and competing cause mortality among women of European ancestry (EAs), its prognostic role among AAs is uncertain, and potential mediating mechanisms have not been elucidated. We propose to evaluate (1) the impact of body fatness (body mass index, body fat distribution, and body composition) on BrCa treatment received (including chemotherapy dose reduction and delay) and outcomes (BrCa-specific mortality, competing-cause mortality, all-cause mortality, quality-of-life); (2) the impact of obesity-related comorbidities which are more common among AAs, including hypertension and diabetes, in relation to BrCa treatment and outcomes, and whether optimal management of these conditions impact outcomes; and (3) potential mechanistic pathways, including immune factors, vitamin D, adipokines, and indicators of insulin resistance that potentially mediate the effects obesity and obesity-related comorbidities. Estimates of genomic ancestry, obtained using genotypes from Ancestry Informative Markers (AIMs) will be included in all analyses. We propose a cost-effective study that builds upon an ongoing population-based case-control study of BrCa in AA women (P01 CA151135). Rapid case ascertainment by the NJ State Cancer Registry (NJSCR) is used to identify newly diagnosed BrCa cases in ten counties in NJ. Here we propose to extend recruitment of AA cases up to March 2018, increasing the total number to ~1,700, with ~1,100 cases with blood samples collected 18 to 24 months after diagnosis and ~860 blood samples collected 30 to 36 months after diagnosis. We propose to establish a cohort of AA BrCa survivors to assess disparities in care, quality-of-life (QoL), and survival, particularly in relation to obesity. A baeline home interview conducted shortly after diagnosis collects information on risk factors, including lifetime weight history. Anthropometric and body composition measurements are taken and a saliva sample is collected. Medical records from multiple providers are obtained and reviewed. We propose to collect blood samples ~12 months and ~24 months after completion of treatment to evaluate obesity-related biomarkers, and to conduct annual active and passive follow-up and obtain detailed medical records. Active follow-up will involve interviews to collect annual updates on anthropometric measurements, diet and physical activity, medical history, including information on comorbidities and their management, as well as survivorship factors (e.g., QoL, lymphedema, perceived stress, and fatigue). Passive follow-up will include record linkage with the NJSCR to obtain vital status and time and causes of death for those who are deceased. Findings from this study have great potential to improve clinical management of obese AA BrCa patients to improve the survival and QoL of BrCa survivors.